do not force external fmt

[rongou]

Checklist

• [x] Used a personal fork of the feedstock to propose changes
• [x] Bumped the build number (if the version is unchanged)
• [x] Re-rendered with the latest conda-smithy (Use the phrase code>@<space/conda-forge-admin, please rerender in a comment in this PR for automated rerendering)
• [x] Ensured the license file is being packaged.

It's a breaking behavior change to force external fmt. If people want to use external fmt, they can still install the fmt conda package and pass in the build flag. This package probably shouldn't enforce that.

[conda-forge-linter]

Hi! This is the friendly automated conda-forge-linting service.

I just wanted to let you know that I linted all conda-recipes in your PR (recipe) and found it was in an excellent condition.

[rongou]

@conda-forge-admin, please rerender